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What is ARQ (automatic repeat request)?

An error-control method in which the receiver, on finding errors or missing data, gets correct data by having the sender send it again. The receiver returns acknowledgments (ACKs) or negative acknowledgments (NAKs), and the sender retransmits if no ACK arrives within a set time. Variants include stop-and-wait, which confirms one unit at a time, Go-Back-N and selective repeat. It is used in TCP, wireless LANs and the data link layer of PCI Express, among others, and is often combined with forward error correction (FEC).

Japanese name: ARQ(自動再送要求)

ARQ (automatic repeat request) appears in 6 practice questions across 3 topics, including Computer Networking Basics, Information Theory and Coding Basics, I/O Interfaces & Buses.

Field: Computer Networking Basics

Try it: questions about ARQ (automatic repeat request)

Which of the following correctly describes ARQ (automatic repeat request) and FEC (forward error correction), methods for dealing with errors in communication?

  1. AARQ corrects errors at the receiver alone, so no channel is needed to send notices back to the sender
  2. BFEC asks for retransmission when it detects an error, so delay tends to grow when there are many errors
  3. CFEC corrects errors at the receiver using redundant bits, so it can also be used for one-way broadcasting
Show answer and explanation

Answer: C. FEC corrects errors at the receiver using redundant bits, so it can also be used for one-way broadcasting

ARQ detects errors with a CRC or similar, tells the sender that the data could not be received and has them sent again; it needs a return channel and is delayed by the retransmissions. FEC sends redundant bits along with the data and corrects errors at the receiver alone, so it suits one-way broadcasting, communication with space probes where a round trip takes a long time, and communication that cannot tolerate delay. Real systems also often combine the two, correcting most errors with FEC and retransmitting the remaining ones with ARQ.

ARQ (automatic repeat request) in the explanations

Questions that mention ARQ (automatic repeat request) (6)

  1. Which of the following correctly describes ARQ (automatic repeat request) and FEC (forward error correction), methods for dealing with errors in communication?
  2. A single even parity bit is added to a bit string of data so that the number of 1s is even, and the receiver checks the number of 1s. Which of the following correctly describes this check?
  3. What does a requirement of "bit error rate (BER) at most 10^-12" for a high-speed serial link mean?
  4. In the OSI reference model, which layer uses logical addresses such as IP addresses to relay packets between different networks until they reach their destination?
  5. Which correctly describes UDP?
  6. What triggers a TCP sender to decide that a segment was lost and retransmit it?

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Related terms

TCP (Transmission Control Protocol) Packet Router Forward error correction (FEC) CRC (cyclic redundancy check) Parity bit Bit error rate (BER) PCI Express OSI reference model IP address

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